Activation of liver myofibroblasts (of different origins) is responsible for the development of liver fibrosis in chronic liver diseases, and remarkably, the clearance of myofibroblasts by apoptosis would allow recovery from liver injury and reversal of liver fibrosis. There is agreement among liver experts that inhibiting or reversing myofibroblastic activation of different cellular origins is critical for the treatment of liver fibrosis. Finally, blocking the progression of liver fibrosis would decrease development of primary liver cancer since the majority of hepatocellular carcinomas arise in cirrhotic livers.
According to the NIH and the WHO (32; 33), the impact of liver diseases can be summarized annually as follows: i) liver cirrhosis: its mortality is approximately 800,000 worldwide (32), and 27,000 in the US; ii) chronic liver diseases: there are 421,000 hospitalizations for chronic liver diseases in the US. In addition, a medication that would prevent progression of liver fibrosis and decrease liver inflammation would impact the management of patients with non-alcoholic steatohepatitis (affects ˜10 million in the US and it is an ‘epidemic’ worldwide); hepatitis C (˜3 million in the US and 170 million worldwide have chronic infection), hepatitis B (˜1 million in the US and 350 million worldwide have chronic infection), as well as those less common chronic liver diseases afflicting adults (primary biliary cirrhosis; sclerosing cholangitis; autoimmune hepatitis; genetic hemochromatosis) and children (including biliary atresia; α-1 antitrypsin deficiency and other rare genetic disorders) for which at present there are no available treatments.
There is no approved medication that directly inhibits or reverses liver fibrosis at present. Current treatments focus on managing the complications that result from liver inflammation and fibrosis. Drug candidates that are in clinical development in this area include: a) GR-MD-02 (Galectin Therapeutics Inc.—Indication—NASH (fatty liver disease) patients with advanced fibrosis—Phase 2). This drug is delivered in liposomes and targets macrophages for apoptosis and not the liver myofibroblasts responsible for the fibrogenesis pathway. Significant off-target adverse effects would be expected since the killing of macrophages could alter the immunological balance; b) Simtuzumab, an antifibrotic monoclonal antibody against lysyl oxidase-like 2 (LOXL2) enzyme (Gilead Sciences-Indications: liver fibrosis; primary sclerosing cholangitis; nonalcoholic steatohepatitis—Phase 2). This drug may prevent progression of active fibrogenesis but will not reverse existing crosslinked collagen fibers. In addition, it may induce immunogenic reactions. The efficacy of a large protein (antibody) is also a concern since it has to interact with LOXL2 in the potentially inaccessible extracellular matrix of a cirrhotic liver; c) Obeticholic acid (OCA) is a bile acid analog and agonist of the farnesoid X receptor (FXR) (Intercept Pharmaceuticals—OCA is being developed for a variety of chronic liver diseases including primary biliary cirrhosis (PBC), nonalcoholic steatohepatitis (NASH), and primary sclerosing cholangitis (PSC)—Phase 3). A major concern is that the blockade of the FXR is associated with the spontaneous development of liver tumors in the absence of the bile acid receptor farnesoid X receptor (26); and d) Emricasan (Conatus Pharmaceuticals—nonalcoholic fatty liver disease (NAFLD) subset of patients with inflammatory and/or fibrotic nonalcoholic steatohepatitis (NASH)—Phase 2). This drug is an active caspase protease inhibitor. A major concern is that prolonged inhibition of hepatocyte caspases may facilitate development of hepatocellular carcinoma and other organ tumors by eliminating a critical anti-tumor check-point (22).
A medication that would decrease or prevent the progression of lung fibrosis would impact the healthcare of patients with Idiopathic Pulmonary Fibrosis (IPF). IPF affects five (5) million people worldwide and 200,000 patients in the US (11). No therapy is known to improve health-related quality of life or survival in patients with IPF and these patients live only 3 to 5 years after diagnosis.
Drug candidates that are in clinical development in this area include: a) Esbriet (pirfenidone) is newly approved by the FDA for the treatment of IPF. However, the product description states that “Esbriet should be used with caution in patients with mild to moderate (Child Pugh Class A and B) hepatic impairment” and also those with mild, moderate or severe renal impairment. The drug may also result in elevated liver enzymes; photosensitivity reaction or rash; gastrointestinal disorders and also drug reactions with concomitant administration with strong inhibitors of CYP1A2 (e.g., fluvoxamine); b) OFEV (nintedanib) is also approved by the FDA for the treatment of IPF. However, the Safety Information Sheet regarding OFEV describes that the therapeutic can cause birth defects or death to an unborn baby, liver problems, bleeding and gastrointestinal disorders, and in more serious cases, stroke and heart attack; c) Oral prednisone (or some other form of corticosteroid) may decrease lung inflammation and the symptoms may improve significantly. The steroids may be used in combination with other drugs. However, the process of benefit to the patients (in terms of results seen) can be slow (1-3 months) and corticosteroids pose the risk of significant side effects; d) Fluimucil (N-acetylcysteine) has been mainly used for symptomatic relief of IPF; however, the supportive palliative care can be costly; e) Cytoxan (cyclophosphamide) may be used for those patients in whom steroid therapy has failed to be effective or is not possible and the drug may also be used as a combination therapeutic with a corticosteroid. The medication is immunosuppressive, and the response to therapy may be slow (6 months or more) and can present significant side effects including bone marrow suppression, blood disorders, and bladder inflammation; to name a few; and f) A combination of prednisone, azathioprine, and N-acetylcysteine (NAC) has been used for the treatment of IPF patients. However, NAC has been seen to be associated with increased risks of death and hospitalization of IPF patients.
Inflammation contributes to the pathogenesis of most acute and chronic liver diseases1. Excessive liver injury and inflammation associated with liver diseases induced by viral, toxic, immunologic, and metabolic diseases2 results in liver dysfunction and in chronic conditions in the potential deposition of scar tissue and the development of cirrhosis, which is in turn a major contributor to the morbidity and mortality of patients affected by chronic liver diseases2, 3. It was reported that amplification of toxic liver injury is mediated by macrophages since TLR-4 ko mice were resistant to hepatotoxins and that reconstitution of bone marrow irradiated TLR-4 ko mice with TLR-4+/+macrophages conferred susceptibility of these animals to hepatotoxins4. The role of macrophages in liver inflammation in toxic liver injury has been confirmed using macrophage ablation5, and further characterized in an experimental alcoholic liver injury model using an IL-1 receptor antagonist6, and in LPS/D-galactosamine induced liver injury using Adenosine-2A (A2A) receptor-ko mice7. Fas-mediated IL-18 secretion from macrophages causes acute liver injury in mice8, and macrophage phagocytosis removes hepatocyte debris during hepatocyte injury9. However, the signal transduction mechanisms in liver macrophages that are indispensable to amplify liver injury have been only partially characterized1.
The inflammasome is a protein complex that is essential for triggering activation of inflammatory reactions in macrophages as well as the consequent macrophage activation1, 10, 11 The CCAAT/Enhancer Binding Protein-β (C/EBPβ)12, 13, 14 has been shown to be a critical signaling molecule for macrophages as expression of a dominant inhibitor of C/EBPβ DNA-binding sites15 or a targeted deletion of C/EBPβ results in impaired macrophage differentiation16.
In addition, C/EBPβ expression is dramatically increased during differentiation of these cells, and is induced by macrophage modulators (LPS, IL-1, G-CSF, TGFβ, vitamin D, retinoic acid)13, 17. In this context, researchers have shown that phosphorylation of C/EBPβ by Ribosomal S-Kinase-2 (RSK-2), which is activated directly by Extracellular-Regulated Kinase (ERK)-½ phosphorylation, plays an essential role in the ERK/Mitogen Activated Protein Kinase (MAPK) signaling pathway regulating cell survival18, 19, 20, 21 Relevant to macrophage activation and survival, it was also reported that expression of the dominant positive, phosphorylation-mutant C/EBPβ-Glu217, which mimics phosphorylated C/EBPβ-Thr217 in biological assays22, was sufficient to rescue the impaired macrophage function and activity induced by Anthrax lethal toxin23.